Imaging protein molecules using FRET and FLIM microscopy

F?rster (or fluorescence) resonance energy transfer (FRET) and fluorescence lifetime imaging (FLIM) have moved center stage and are increasingly forming part of multifaceted imaging approaches. They are complementary methodologies that can be applied to advanced quantitative analyses. The widening application of FRET and FLIM has been driven by the availability of suitable fluorophores, increasingly sophisticated microscopy systems, methodologies to correct spectral bleed-through, and the ease with which FRET can be combined with other techniques. FRET and FLIM have recently found use in several applications: in the analysis of protein-protein interactions with high spatial and temporal specificity (e.g. clustering), in the study of conformational changes, in the analysis of binding sequences, and in applications such as high-throughput screening.     

Development of a novel FRET immunosensor technique

We report on a novel technique to develop an optical immunosensor based on fluorescence resonance energy transfer (FRET). IgG antibodies were labeled with acceptor fluorophores while one of three carrier molecules (protein A, protein G, or F(ab')2 fragment) was labeled with donor fluorophores. The carrier molecule was incubated with the antibody to allow specific binding to the Fc portion. The labeled antibody-protein complex was then exposed to specific and nonspecific antigens, and experiments were designed to determine the 'in solution' response. The paper reports the results of three different donor-acceptor FRET pairs, fluorescein isothiocyanate/tetramethylrhodamine isothiocyanate, Texas Red/Cy5, and Alexa Fluor 546/Alexa Fluor 594. The effects of the fluorophore to protein conjugation ratio (F/P ratio) and acceptor to donor fluorophore ratios between the antibody and protein (A/D ratio) were examined. In the presence of specific antigens, the antibodies underwent a conformational change, resulting in an energy transfer from the donor to the acceptor fluorophore as measured by a change in fluorescence. The non-specific antigens elicited little or no changes. The Alexa Fluor FRET pair demonstrated the largest change in fluorescence, resulting in a 35% change. The F/P and A/D ratio will affect the efficiency of energy transfer, but there exists a suitable range of A/D and F/P ratios for the FRET pairs. The feasibility of the FRET immunosensor technique was established; however, it will be necessary to immobilize the complexes onto optical substrates so that consistent trends can be obtained that would allow calibration plots.     

Elastic deformations of the rotary double motor of single F(o)F(1)-ATP synthases detected in real time by Förster resonance energy transfer

Elastic conformational changes of the protein backbone are essential for catalytic activities of enzymes. To follow relative movements within the protein, F?rster-type resonance energy transfer (FRET) between two specifically attached fluorophores can be applied. FRET provides a precise ruler between 3 and 8nm with subnanometer resolution. Corresponding submillisecond time resolution is sufficient to identify conformational changes in FRET time trajectories. Analyzing single enzymes circumvents the need for synchronization of various conformations. F(O)F(1)-ATP synthase is a rotary double motor which catalyzes the synthesis of adenosine triphosphate (ATP). A proton-driven 10-stepped rotary F(O) motor in the Escherichia coli enzyme is connected to a 3-stepped F(1) motor, where ATP is synthesized. To operate the double motor with a mismatch of step sizes smoothly, elastic deformations within the rotor parts have been proposed by W. Junge and coworkers. Here we extend a single-molecule FRET approach to observe both rotary motors simultaneously in individual F(O)F(1)-ATP synthases at work. We labeled this enzyme with two fluorophores specifically, that is, on the ε- and c-subunits of the two rotors. Alternating laser excitation was used to select the FRET-labeled enzymes. FRET changes indicated associated transient twisting within the rotors of single enzyme molecules during ATP hydrolysis and ATP synthesis. Supported by Monte Carlo simulations of the FRET experiments, these studies reveal that the rotor twisting is greater than 36° and is largely suppressed in the presence of the rotation inhibitor DCCD. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).     

ATP-induced transconformation of myosin revealed by determining three-dimensional positions of fluorophores from fluorescence energy transfer measurements

The method of fluorescence resonance energy transfer (FRET) is one of the most important techniques for measuring the distance between two fluorophores and for detecting the changes in protein structure under physiological conditions. The use of green fluorescent protein is also a powerful technology that has been used to elucidate dynamic molecular events. From these we have developed a novel method to determine the three-dimensional positions of fluorophores by combining the FRET data and other structural information available. Using this method, we could determine the ATP-induced changes of three-dimensional structure of truncated Dictyostelium myosin in solution. The myosin structure with ADP in solution was found to be similar to that of the crystal structure of MgADPBeFx-bound truncated Dictyostelium myosin (type I structure), whereas myosin with ATP in solution was similar to the crystal structure of MgAdPVi-bound one (type II structure). However, the crystal structure of MgADP-bound scallop myosin (type III structure) could not be explained by any of our FRET data under various conditions. This indicates that the type III crystal structure might represent a transient intermediate conformation that could not be detected using fluorescence energy transfer.     

Fluorescence resonance energy transfer-based intracellular assay for the conformation of hepatitis C virus drug target NS5A

Nonstructural protein 5A (NS5A) is essential for hepatitis C virus (HCV) replication and assembly and is a critical drug target. Biochemical data suggest large parts of NS5A are unfolded as an isolated protein, but little is known about its folded state in the cell. We used fluorescence resonance energy transfer (FRET) to probe whether or not different regions of NS5A are in close proximity within the cell. Twenty-three separate reporter constructs were created by inserting one or more fluorophores into different positions throughout the three domains of NS5A. FRET efficiency was maximal when donor and acceptor fluorophores were positioned next to each other but also could be observed when the two fluorophores flanked NS5A domain 1 or domain 3. Informatic and biochemical analysis suggests that large portions of the carboxy terminus of NS5A are in an unfolded and disordered state. Quercetin, a natural product known to disrupt NS5A function in cells, specifically disrupted a conformationally specific domain 3 FRET signal. Intermolecular FRET indicated that the NS5A amino termini, but not other regions, are in close proximity in multimeric complexes. Overall, this assay provides a new window on the intracellular conformation(s) of NS5A and how the conformation changes in response to cellular and viral components of the replication and assembly complex as well as antiviral drugs.     

Fluorescent proteins for FRET microscopy: monitoring protein interactions in living cells

The discovery and engineering of novel fluorescent proteins (FPs) from diverse organisms is yielding fluorophores with exceptional characteristics for live-cell imaging. In particular, the development of FPs for fluorescence (or F?rster) resonance energy transfer (FRET) microscopy is providing important tools for monitoring dynamic protein interactions inside living cells. The increased interest in FRET microscopy has driven the development of many different methods to measure FRET. However, the interpretation of FRET measurements is complicated by several factors including the high fluorescence background, the potential for photoconversion artifacts and the relatively low dynamic range afforded by this technique. Here, we describe the advantages and disadvantages of four methods commonly used in FRET microscopy. We then discuss the selection of FPs for the different FRET methods, identifying the most useful FP candidates for FRET microscopy. The recent success in expanding the FP color palette offers the opportunity to explore new FRET pairs.     

VenusA206 Dimers Behave Coherently at Room Temperature

Fluorescent proteins (FPs) have revolutionized cell biology by allowing genetic tagging of specific proteins inside living cells. In conjunction with Förster’s resonance energy transfer (FRET) measurements, FP-tagged proteins can be used to study protein-protein interactions and estimate distances between tagged proteins. FRET is mediated by weak Coulombic dipole-dipole coupling of donor and acceptor fluorophores that behave independently, with energy hopping discretely and incoherently between fluorophores. Stronger dipole-dipole coupling can mediate excitonic coupling in which excitation energy is distributed near instantaneously between coherently interacting excited states that behave as a single quantum entity. The interpretation of FP energy transfer measurements to estimate separation often assumes that donors and acceptors are very weakly coupled and therefore use a FRET mechanism. This assumption is considered reasonable as close fluorophore proximity, typically associated with strong excitonic coupling, is limited by the FP β-barrel structure. Furthermore, physiological temperatures promote rapid vibrational dephasing associated with a rapid decoherence of fluorophore-excited states. Recently, FP dephasing times that are 50 times slower than traditional organic fluorophores have been measured, raising the possibility that evolution has shaped FPs to allow stronger than expected coupling under physiological conditions. In this study, we test if excitonic coupling between FPs is possible at physiological temperatures. FRET and excitonic coupling can be distinguished by monitoring spectral changes associated with fluorophore dimerization. The weak coupling mediating FRET should not cause a change in fluorophore absorption, whereas strong excitonic coupling causes Davydov splitting. Circular dichroism spectroscopy revealed Davydov splitting when the yellow FP Venus_A206 dimerizes, and a novel approach combining photon antibunching and fluorescence correlation spectroscopy was used to confirm that the two fluorophores in a Venus_A206 homodimer behave as a single-photon emitter. We conclude that excitonic coupling between Venus_A206 fluorophores is possible at physiological temperatures.     

An ion-insensitive cAMP biosensor for long term quantitative ratiometric fluorescence resonance energy transfer (FRET) measurements under variable physiological conditions

Ratiometric measurements with FRET-based biosensors in living cells using a single fluorescence excitation wavelength are often affected by a significant ion sensitivity and the aggregation behavior of the FRET pair. This is an important problem for quantitative approaches. Here we report on the influence of physiological ion concentration changes on quantitative ratiometric measurements by comparing different FRET pairs for a cAMP-detecting biosensor. We exchanged the enhanced CFP/enhanced YFP FRET pair of an established Epac1-based biosensor by the fluorophores mCerulean/mCitrine. In the case of enhanced CFP/enhanced YFP, we showed that changes in proton, and (to a lesser extent) chloride ion concentrations result in incorrect ratiometric FRET signals, which may exceed the dynamic range of the biosensor. Calcium ions have no direct, but an indirect pH-driven effect by mobilizing protons. These ion dependences were greatly eliminated when mCerulean/mCitrine fluorophores were used. For such advanced FRET pairs the biosensor is less sensitive to changes in ion concentration and allows consistent cAMP concentration measurements under different physiological conditions, as occur in metabolically active cells. In addition, we verified that the described FRET pair exchange increased the dynamic range of the FRET efficiency response. The time window for stable experimental conditions was also prolonged by a faster biosensor expression rate in transfected cells and a greatly reduced tendency to aggregate, which reduces cytotoxicity. These properties were verified in functional tests in single cells co-expressing the biosensor and the 5-HT(1A) receptor.     

利用荧光寿命变化定量分子内和分子间相互作用的方法

荧光寿命是指荧光分子在回到基态前在激发态停留的平均时间.本文发展了基于荧光寿命测量来定量分子内和分子间相互作用的方法:通过G碱基猝灭对于荧光寿命的影响定量DNA二级结构的形成;通过荧光共振能量传递(FRET)中荧光寿命的变化来定量分子间的相互作用.第一种方法巧妙利用了G碱基会猝灭临近的染料分子的性质,结合荧光寿命的变化,可以判断DNA二级结构的形成以及形成的比率. FRET是用于研究生物分子相互作用的一个重要手段.传统的FRET方法主要是基于强度的变化,但这种变化容易受到荧光表达水平变动、样品中分子扩散以及荧光串色的影响,因此经常存在着实验比较复杂和重复性差的问题.而基于荧光寿命的FRET研究则可以很好地克服上述缺点.通过检测供体荧光寿命的变化,我们能够方便快捷地判断是否发生FRET,并通过建立系统的数据分析方法,得到FRET的效率以及分子之间相互作用的信息.     

Correcting confocal acquisition to optimize imaging of fluorescence resonance energy transfer by sensitized emission

Imaging of fluorescence resonance energy transfer (FRET) between suitable fluorophores is increasingly being used to study cellular processes with high spatiotemporal resolution. The genetically encoded Cyan (CFP) and Yellow (YFP) variants of Green Fluorescent Protein have become the most popular donor and acceptor pair in cell biology. FRET between these fluorophores can be imaged by detecting sensitized emission. This technique, for which CFP is excited and transfer is detected as emission of YFP, is sensitive, fast, and straightforward, provided that proper corrections are made. In this study, the detection of sensitized emission between CFP and YFP by confocal microscopy is optimized. It is shown that this FRET pair is best excited at 430 nm. We identify major sources of error and variability in confocal FRET acquisition including chromatic aberrations and instability of the excitation sources. We demonstrate that a novel correction algorithm that employs online corrective measurements yields reliable estimates of FRET efficiency, and it is also shown how the effect of other error sources can be minimized.     

Modelling Förster resonance energy transfer (FRET) using anisotropy resolved multi-dimensional emission spectroscopy (ARMES)

BackgroundFörster Resonance Energy Transfer (FRET) is widely used to study the structure and dynamics of biomolecular systems and also causes the non-linear fluorescence response observed in multi-fluorophore proteins. Accurate FRET analysis, in terms of measuring changes in donor and acceptor spectra and energy transfer efficiency is therefore critical.MethodsWe demonstrate a novel quantitative FRET analysis using anisotropy resolved multidimensional emission spectroscopy (ARMES) in a Human Serum Albumin (HSA) and 1,8-anilinonaphathalene sulfonate (ANS) model. ARMES combines 4D measurement of polarized excitation emission matrices (pEEM) with multivariate data analysis to spectrally resolve contributing fluorophores. Multivariate analysis (Parallel Factor, PARAFAC and restricted Tucker3) was used to resolve fluorophore contributions and for modelling the quenching of HSA emission and the HSA-ANS interactions.ResultspEEM spectra were modelled using Tucker3 which accommodates non-linearities introduced by FRET and a priori chemical knowledge was used to optimise the solution, thus resolving three components: HSA emission, ANS emission from indirect FRET excitation, and ANS emission from direct excitation. Perpendicular emission measurements were more sensitive to indirectly excited acceptor emission. PARAFAC modelling of HSA, donor emission, separated ANS FRET interacting (Tryptophan) and non-interacting (Tyrosine) components. This enabled a new way of calculating quenching constants using the multi-dimensional emission of individual donor fluorophores.ConclusionsFRET efficiency could be calculated using the multi-dimensional, resolved emission of the interacting donor fluorophores only which yielded higher ET efficiencies compared to conventional methods.General significanceShows the potential of multidimensional fluorescence measurements and data analysis for more accurate FRET modelling in proteins.     

FRET between cardiac Na+ channel subunits measured with a confocal microscope and a streak camera

When and where proteins associate is a central question in many biomolecular studies. F?rster resonance energy transfer (FRET) measurements can be used to address this question when the interacting proteins are labeled with appropriate donor and acceptor fluorophores. We describe an improved method to determine FRET efficiency that uses a mode-locked laser, a confocal microscope and a streak camera. We applied this method to study the association of alpha and beta(1) subunits of the human cardiac sodium channel. The subunits were tagged with the cyan and yellow variants of the green fluorescent protein (GFP) and expressed in human embryonic kidney (HEK293) cells. Pronounced FRET between the channel subunits in the endoplasmic reticulum (ER) suggested that the subunits associate before they reach the plasma membrane. The described method allows simultaneous measurement of donor and acceptor fluorescence decays and provides an intrinsically validated estimate of FRET efficiency.     

Imaging protein-protein interactions using fluorescence resonance energy transfer microscopy

Fluorescence resonance energy transfer (FRET) detects the proximity of fluorescently labeled molecules over distances >100 A. When performed in a fluorescence microscope, FRET can be used to map protein-protein interactions in vivo. We here describe a FRET microscopy method that can be used to determine whether proteins that are colocalized at the level of light microscopy interact with one another. This method can be implemented using digital microscopy systems such as a confocal microscope or a wide-field fluorescence microscope coupled to a charge-coupled device (CCD) camera. It is readily applied to samples prepared with standard immunofluorescence techniques using antibodies labeled with fluorescent dyes that act as a donor and acceptor pair for FRET. Energy transfer efficiencies are quantified based on the release of quenching of donor fluorescence due to FRET, measured by comparing the intensity of donor fluorescence before and after complete photobleaching of the acceptor. As described, this method uses Cy3 and Cy5 as the donor and acceptor fluorophores, but can be adapted for other FRET pairs including cyan fluorescent protein and yellow fluorescent protein.     

A novel analytical method for in vivo phosphate tracking

Genetically-encoded fluorescence resonance energy transfer (FRET) sensors for phosphate (P(i)) (FLIPPi) were engineered by fusing a predicted Synechococcus phosphate-binding protein (PiBP) to eCFP and Venus. Purified fluorescent indicator protein for inorganic phosphate (FLIPPi), in which the fluorophores are attached to the same PiBP lobe, shows P(i)-dependent increases in FRET efficiency. FLIPPi affinity mutants cover P(i) changes over eight orders of magnitude. COS-7 cells co-expressing a low-affinity FLIPPi and a Na(+)/P(i) co-transporter exhibited FRET changes when perfused with 100 microM P(i), demonstrating concentrative P(i) uptake by PiT2. FLIPPi sensors are suitable for real-time monitoring of P(i) metabolism in living cells, providing a new tool for fluxomics, analysis of pathophysiology or changes of P(i) during cell migration.     

Single-Molecule Three-Color FRET with Both Negligible Spectral Overlap and Long Observation Time

Full understanding of complex biological interactions frequently requires multi-color detection capability in doing single-molecule fluorescence resonance energy transfer (FRET) experiments. Existing single-molecule three-color FRET techniques, however, suffer from severe photobleaching of Alexa 488, or its alternative dyes, and have been limitedly used for kinetics studies. In this work, we developed a single-molecule three-color FRET technique based on the Cy3-Cy5-Cy7 dye trio, thus providing enhanced observation time and improved data quality. Because the absorption spectra of three fluorophores are well separated, real-time monitoring of three FRET efficiencies was possible by incorporating the alternating laser excitation (ALEX) technique both in confocal microscopy and in total-internal-reflection fluorescence (TIRF) microscopy.     

Fluorescence-based glucose sensors

There is an urgent need to develop technology for continuous in vivo glucose monitoring in subjects with diabetes mellitus. Problems with existing devices based on electrochemistry have encouraged alternative approaches to glucose sensing in recent years, and those based on fluorescence intensity and lifetime have special advantages, including sensitivity and the potential for non-invasive measurement when near-infrared light is used. Several receptors have been employed to detect glucose in fluorescence sensors, and these include the lectin concanavalin A (Con A), enzymes such as glucose oxidase, glucose dehydrogenase and hexokinase/glucokinase, bacterial glucose-binding protein, and boronic acid derivatives (which bind the diols of sugars). Techniques include measuring changes in fluorescence resonance energy transfer (FRET) between a fluorescent donor and an acceptor either within a protein which undergoes glucose-induced changes in conformation or because of competitive displacement; measurement of glucose-induced changes in intrinsic fluorescence of enzymes (e.g. due to tryptophan residues in hexokinase) or extrinsic fluorophores (e.g. using environmentally sensitive fluorophores to signal protein conformation). Non-invasive glucose monitoring can be accomplished by measurement of cell autofluorescence due to NAD(P)H, and fluorescent markers of mitochondrial metabolism can signal changes in extracellular glucose concentration. Here we review the principles of operation, context and current status of the various approaches to fluorescence-based glucose sensing.     

Construction and optimization of a family of genetically encoded metabolite sensors by semirational protein engineering

A family of genetically-encoded metabolite sensors has been constructed using bacterial periplasmic binding proteins (PBPs) linearly fused to protein fluorophores. The ligand-induced conformational change in a PBP allosterically regulates the relative distance and orientation of a fluorescence resonance energy transfer (FRET)-compatible protein pair. Ligand binding is transduced into a macroscopic FRET observable, providing a reagent for in vitro and in vivo ligand-measurement and visualization. Sensors with a higher FRET signal change are required to expand the dynamic range and allow visualization of subtle analyte changes under high noise conditions. Various observations suggest that factors other than inter-fluorophore separation contribute to FRET transfer efficiency and the resulting ligand-dependent spectral changes. Empirical and rational protein engineering leads to enhanced allosteric linkage between ligand binding and chromophore rearrangement; modifications predicted to decrease chromophore rotational averaging enhance the signal change, emphasizing the importance of the rotational freedom parameter kappa2 to FRET efficiency. Tighter allosteric linkage of the PBP and the fluorophores by linker truncation or by insertion of chromophores into the binding protein at rationally designed sites gave rise to sensors with improved signal change. High-response sensors were obtained with fluorescent proteins attached to the same binding PBP lobe, suggesting that indirect allosteric regulation during the hinge-bending motion is sufficient to give rise to a FRET response. The optimization of sensors for glucose and glutamate, ligands of great clinical interest, provides a general framework for the manipulation of ligand-dependent allosteric signal transduction mechanisms.     

Elucidating mechanistic principles underpinning eukaryotic translation initiation using quantitative fluorescence methods

Eukaryotic translation initiation is an intricate process involving at least 11 formally classified eIFs (eukaryotic initiation factors), which, together with the ribosome, comprise one of the largest molecular machines in the cell. Studying such huge macromolecular complexes presents many challenges which cannot readily be overcome by traditional molecular and structural methods. Increasingly, novel quantitative techniques are being used to further dissect such complex assembly pathways. One area of methodology involves the labelling of ribosomal subunits and/or eIFs with fluorophores and the use of techniques such as FRET (F?rster resonance energy transfer) and FA (fluorescence anisotropy). The applicability of such techniques in such a complex system has been greatly enhanced by recent methodological developments. In the present mini-review, we introduce these quantitative fluorescence methods and discuss the impact they are beginning to have on the field.     

Development of dual receptor biosensors: an analysis of FRET pairs

The development of a dual receptor detection method for enhanced biosensor monitoring was investigated by analyzing potential fluorescent resonance energy transfer (FRET) pairs. The dual receptor scheme requires the integration of a chemical transducer system with two unique protein receptors that bind to a single biological agent. The two receptors are tagged with special molecular groups (donors and acceptors fluorophores) while the chemical transduction system relies on the well-known mechanisms of FRET. During the binding event, the two FRET labeled receptors dock at the binding sites on the surface of the biological agent. The resulting close proximity of the two fluorophores upon binding will initiate the energy transfer resulting in fluorescence. The paper focuses on the analysis and optimization of the chemical transduction system. A variety of FRET fluorophore pairs were tested in a spectrofluorimeter and promising FRET pairs were then tagged to the protein, avidin and its ligand, biotin. Due to their affinities, the FRET-tagged biomolecules combine in solution, resulting in a stable, fluorescent signal from the acceptor FRET dye with a simultaneous decrease in fluorescent signal from the donor FRET dye. The results indicate that the selected FRET pairs can be utilized in the development of dual receptor sensors.